The positron emission tomography (PET) has received attention as being effective for early diagnosis of cancer. The PET or an examination method has been used for examining the presence or level of diseases or for cancer diagnosis by giving a compound marked by a trace amount of a positron emission nuclide to the body and then detecting annihilation radiation emitted therefrom, thereby imaging a metabolic function such as glucose metabolism. In order to implement this examination method, the PET device has been brought into practical use.
The principle of the PET is as described below. Positrons emitted in the positron decay of a positron emission nuclide may disappear by annihilation in pairs with surrounding electrons so as to yield a pair of annihilation radiations at 511 keV, which are measured with a pair of radiation detectors on the basis of the principle of coincidence. This makes it possible to identify the position of presence of the nuclide on one line segment connecting between the pair of detectors (a line of response). The distribution of nuclides within the patient body can be known from the data obtained by measuring lines of response in various directions using the detectors disposed so as to surround the patient and observing PET images provided by an image reconstruction operation. The research and development of new PET medicines (probes) for diagnosis of cancer properties, typified by an oxygen state, have been actively conducted in addition to the research and development for providing improved device performances such as resolution.
Nuclear medicine imaging apparatus including the PET measure radiation in a pulse mode in which the radiation is measured (or counted) on every pulse.
On the other hand, the role of therapy for the cancer that is identified by diagnosis with the PET is also critical. As a method different from surgery or medication, there is available a radiation therapy in which an affected area is irradiated with radiation such as X-rays or gamma rays. In particular, the particle beam therapy in which a cancer portion is concentratedly irradiated with heavy particle beams or proton beams has gained great attention as a method which offers outstanding therapeutic effects and allows for irradiating affected areas with sharply focused beams. As a method for irradiation with particle beams, studies have been conducted on spot scanning irradiation for scanning a pencil beam across an affected area, e.g., to follow the shape thereof, in addition to the conventional Bolus irradiation for spreading the beam with which the affected area is irradiated so as to follow the shape thereof (Non-Patent Literature 1). Any of the studies above are conducted by providing precise control to the direction and dose of irradiation beams in accordance with the therapy plan which has been carefully computed on the basis of a separately captured X-ray CT image or the like. However, there is no denying the risk that a tumor would vary in shape in several weeks from the creation of a therapy plan to the practicing of the therapy, and there is no way for checking whether irradiations have been performed as planned, under current circumstances except for prognostic diagnosis after several weeks.
In this context, the applicants have tried integrating the therapy apparatus with the PET device so as to enable a therapy plan itself to be immediately modified on the basis of a PET image, whereby it has been aimed to achieve the positive radiation cancer therapy which is optimized for each patient and thus each tumor by performing irradiation while (1) directly observing the cancer, (2) observing the dose distribution, and also (3) observing the therapeutic effects. More specifically, as shown in FIG. 1, as a method for enabling three-dimensional PET imaging with a gap through which a therapeutic beam passes, the applicants have suggested the open PET device in which multi-ring detectors 22 and 24 divided into two in the direction of the body axis of a patient 8 (along the z axis in the figure) are spaced apart from each other and which has a physically opened field of view region (also referred to as the open field of view) (Patent Literature 1 and Non-Patent Literature 2). In the open field of view, an image is reconstructed from the lines of response between both the divided detector rings 22 and 24. The figure shows a bed 10, a bed base 12, a gantry cover 26, a radiating apparatus 30, and a therapeutic beam 32.
There have been available previous examples in which a counter (dual) gamma camera type PET device specialized in two-dimensional imaging was combined with a radiation therapy. (1) Concerning the irradiation while directly observing the cancer, studies have been conducted on a method of directly visualizing a tumor referring to not a conventional X-ray transmission image but a PET image in aligning the patient (Non-Patent Literature 3). Furthermore, (2) concerning the observation of the dose distribution, studies have been conducted on a method in which the PET medicine is not given, but in the irradiation with particle beams or X-ray, the annihilation radiation to be produced through the fragmentation reaction of an incident nucleus, the fragmentation reaction of a target nucleus (also referred to as the auto activation), or the photonuclear reaction is imaged on the basis of the principle of the PET (Non-Patent Literatures 4 and 5). Therapy monitoring is considered possible because the position of occurrence of the annihilation radiation is strongly correlated with the dose distribution of irradiation beams.
However, reviewing the processing from the measurement to imaging of radiation, it took several minutes to compute a reconstructed image. Thus, in any conventional methods, it was impossible to modify the therapy plan in synchronization with the therapy on the basis of the information obtained from the PET image and then control the irradiation beam. That is, to implement the beam control based on the feedback from the PET image, it is required to implement high-speed imaging nearly at a real-time level.
The image reconstruction technique is largely divided into the analytical image reconstruction technique which is typified by the filtered back-projection method, and the iterative image reconstruction technique which is typified by the maximum-likelihood expectation-maximization (ML-EM) method. The former can perform calculations quickly but there is a limit in improving image quality. The latter is known to be effective for improving image quality, but requires a long time for iterative computations; attention is being focused on a high-speed method, such as the OSEM method (Non-Patent Literature 6), in which data is divided into subsets (blocks) and then images are updated in blocks. In previous studies based on the same way of thinking as for the X-ray CT, the sinogram (the histogram data of measured counts) was divided into blocks, and thus the number of blocks was limited and the level of speed enhancement was far off the real-time level. On the other hand, the real time processing is impossible in the first place even if images can be reconstructed at sufficiently enhanced speeds because measured counts have to be integrated over time before the sinogram is obtained. However, for the PET, if images can be directly reconstructed from the list mode data (the list of data (count data) on each count of annihilation radiation), which is the source of sinogram, the aforementioned integration over time is not required and the number of blocks can be increased to a great extent. Thus, a significant enhancement in the speed of reconfiguration computations can be expected (Non-Patent Literature 7). This image reconstruction technique shows great promise for the possibility of providing a reconstructed image through only one computation (one-pass) without iterative computations.